It has become increasingly important to be able to detect, measure and remove very low concentrations of contaminants in deionized water with a high degree effectiveness and reproducibility of results. For municipal water supplies, certain maximum contaminant levels (e.g., 500 ppb for boron) must not be exceeded, while for certain industrial applications, such as semiconductor manufacture, levels below about 100 ppt are desirable. This is because even very low levels of boron present in the deionized UPW product water used in manufacturing can significantly and adversely affect the quality and performance of a semiconductor chip.
Large amounts of ultrapure water are required in processes to manufacture semiconductors, and boron may be present as a contaminant in the raw or pretreated feed water. If present, it must be removed to very low concentration, for the following reason. Boron is a p-type semiconductor dopant used in manufacture of solid state electronics, and it functions as a principal charge carrier in the doped silicon crystal. The presence of boron even at a sub ppb level in a fab plant process fluid, such as developer, cleaning fluid, vapor, rinse water or the like can give rise to surface deposits of boron, which in turn, may become incorporated in a silicon substrate during various process stages—particularly heating or ion implantation stages, and may change the intended dopant profile or otherwise alter the electrical characteristics of the substrate. To prevent the inadvertent introduction of boron contamination during manufacturing processes, it is necessary to remove boron from the fab plant UPW stream down to a very low residual level, typically to a low threshold under 50 ppt, and preferably below 10-20 ppt.
Several general facts about the sources of and levels of boron present in natural waters, and its passage in treated waters, affect the ability of a treatment plant to dependably or economically achieve a product level below a set level. Boron may be present in local water sources at levels of about 50 ppb to several ppm (or more for sea water), so it is sometimes necessary when treating bulk water to carry out specific steps to remove boron to a desired level. One level, in order to meet drinking water specifications, may be between 50-500 ppb; this may require a 10- to 100-fold reduction for a seawater reverse osmosis plant. Another level, suitable for certain agricultural applications (where particular crops may require a ceiling) is several hundred ppb, and this target may be set for a plant treating municipal waste, agricultural run-off or brackish groundwater, among other sources.
Several water treatment technologies achieve some reduction of boron, and may be applied to meet such standards. These include reverse osmosis, various complexing agents followed by filtration, as well as boron selective resins and other treatment approaches. Water treatment plants typically involve a pretreatment stage followed by a sequence of other processes, possibly with return loops and bleed or blowdown exit points, bypasses and/or blending to meet diverse competing goals of water recovery, economy, and safe waste disposal.
A UPW treatment plant for semiconductor fabrication processes must generally demineralize the feed water down to a residual boron concentration in the product in the low parts per trillion, the precise level often being specified for expedience by the boron detection limit of the instrumentation available at the plant. Such a level shall for simplicity be referred to herein simply as “boron free”.
Boron is generally present in water in a form that is poorly ionized at middle pH, and when relying on a reverse osmosis (RO) treatment, about 50-70% of the starting boron level may be expected to pass in the permeate. Much greater removal is possible by raising the pH substantially, e.g., to about pH 11, and cuts of 98-99% may be achieved with RO when the ionic load and membrane characteristics permit an RO unit to be operated in this condition.
Ion exchange is of limited effect. Because boron is poorly captured and is loosely held, it may be said that ion exchange doesn't really stop boron passage, it just slows it down. Generally, boron is poorly ionized at neutral pH, and is poorly captured and is weakly held by ion exchange resins. While it may be effectively captured by fully regenerated resin, it may be eluted by other elements (including OH from the equilibrium dissociation of water) as ionic load in the exchange bed increases. Boron ions are therefore the first ions to leak from the mixed ion exchange resin beds that follow RO or that in some areas constitute a principal pretreatment. Leakage can occur as the resin becomes exhausted, and will also occur (in the case of boron) in pulses or spikes much earlier, as bleeding due to displacement by an influx of competing ions, or due to a change in temperature, pH or the like.
System design must therefore address the dependable and effective removal of boron to a predetermined level despite the relatively wide range of possible starting concentrations and competing minerals, the seasonal or episodic changes in source quality and composition, and the relatively variable rates of boron removal or retention using standard water treatment processes.
An illustration is instructive. S. Malhotra et al. have reported in “Correlation of Boron Breakthrough versus Resistivity and Dissolved Silica in RO/DI System” (Ultrapure Water, May/June 1996. 13(4): p. 22-26) that boron was the first ion to break through the ion exchange resin beds of a water treatment system when they switched to using thin-film-composite (TFC) membranes in their reverse osmosis (RO) units. The introduction of TFC reverse osmosis (RO) membranes (to replace cellulose acetate RO membranes) was very effective in reducing the silica passage through the RO apparatus, but the reduction in boron passage was not as great. This led to quicker boron saturation of the ion exchange beds and the observation of unexpected boron breakthrough in a mixed ion exchange resin bed. More generally, once boron has accumulated in an ion exchange bed, changes or upsets in operating conditions may lead to boron release at unacceptable levels. For example, an increase in temperature may result in release of the captured boron (apparently displaced by the higher levels of thermally dissociated hydroxyl ions); similarly, an increase in the level of one or more other dissolved components in the feed water may displace some of the captured boron, potentially eluting higher concentrations of boron than were present in the feed. Thus, standard ion exchange resins are ill-adapted to producing boron-free water.
Adsorption of borate ion on anion exchange resin or selective boron capture resin is the most common method to for producing boron free water. Several systems incorporating this approach have been described in the patent literature, such as those described in U.S. Pat. Nos. 5,811,012 and 5,833,846 of M. Tanabe et al. Those patents show boron-specific ion exchange resin downstream of a degasifier and upstream of a final mixed resin bed, and the patentee reports boron measurements by ICP-MS of below 10 ppt (apparently its limit of detection). One well-known boron-specific exchange resin that may be used in such applications is Amberlite IRA-743T, manufactured by Rohm and Haas Company. Capture resins are widely used at various stages in other treatment systems to meet a required level.
However, boron-specific removal resins generally shed organic carbon, and for semiconductor applications they must therefore be situated upstream of other removal processes. To minimize expense, such treatment may be implemented on a smaller scale in a treatment branch process to supply water only for the specific fab processes where boron affects product quality. However, these resins, like other exchange resins, necessarily require the use of hazardous chemicals to regenerate the resins, so their use raises certain environmental or safety (as well as related cost) concerns. Moreover, capture resins cost several times as much as other exchange resins (e.g., about $500-700 dollars per cubic foot).
When the product water must meet a predetermined maximum contaminant level (MCL) or threshold, particularly when the threshold is very low or the feed is variable, system design may be difficult or present only costly solutions. It is possible that boron removal at higher levels may be enhanced by certain operating protocols with an RO treatment unit that would allow a downstream process, such as a polishing loop with virgin ion exchange resin or a primary makeup loop with ion exchange bottles or capture resin, to effectively remove residual boron (and other material) to sub ppb levels with a reasonable resin lifetime between replacements or regeneration. However, identifying treatment conditions or configurations of treatment units that will have such high levels of removal and operate stably to produce product water below a predetermined threshold, or to produce UPW fab water with boron at a low ppt level, remains a problem. Use of such systems when chip production valued at tens of thousands of dollars per hour are at risk may appear to require substantial verification of the process, or fail safe operating protocols before gaining acceptance in the industry. For systems meeting a higher, less stringent, threshold, the above-described properties still make dependable modeling difficult.
Thus, there remains a need for systems and methods that operate to continuously and effectively produce UPW product water having a boron level below a predetermined level.
There is also a need for water treatment methods and systems to produce UPW water for semiconductor manufacture and other applications, which remove boron down to very low levels without the use of hazardous chemicals.
In addition there is a need for systems and methods to detect change or upset in a UPW treatment unit, and which control a treatment unit or its operating environment to maintain quality of the feed water.
Accordingly, a principal object of this invention is to provide methods and apparatus for water treatment, wherein the treatment system has a boron output below a predetermined maximum level and employs at least one RO treatment unit and/or filled cell ED/EDI unit.
It is a general object of this invention to provide methods and apparatus for water treatment in semiconductor manufacturing or other application which require accurate, reliable removal of boron to a low concentration, e.g., below a predetermined threshold that is between 0 and 500 ppb boron, wherein the system effects a boron detection or measurement, and applies such measurement to control or regulate a condition of operation of a filled cell electrodialysis stack, an RO unit or a related flow treatment or flow conditioning unit.
It is also an object of this invention to control a treatment system in accordance with a boron detection and measurement device, applying a boron level measurement to regulate the electrical current or voltage supplied to a filled cell electrodialysis stack and/or system.
As applied to an RO system, an object of this invention is to provide methods and apparatus for controlling an operating condition of an RO unit in accordance with a system by sampling water with a boron detection/measurement device and applying the measurement to regulate the operating condition of an RO unit.
Other objects and advantages of the present invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises, but is not limited to, the methods and related apparatus, involving the several steps and the various components, and the relation and order of one or more such steps and components with respect to each of the others, as exemplified by the following description and the accompanying drawings.